The present invention relates to a method of fabricating a flash memory device and, more particularly, to a method of fabricating a flash memory device, which can prevent the formation of a bridge between neighboring drain contact plugs.
In general, a cell array of a NAND flash device includes a plurality of cell blocks. The cell array of each cell block is divided into strings unlike a typical flash device. A drain contact and a source contact for global ground, which are connected by a bit line, are disposed at opposite ends of the string. The contacts are connected to a junction of a select transistor for string control.
A process of forming the drain contact plug of a conventional flash device is described below. A plurality of string structures, including a source select transistor, a plurality of memory cells and a drain select transistor, are formed in a semiconductor substrate. A source contact plug is formed through a specific process. An interlayer dielectric layer is formed on the entire surface of the semiconductor substrate. Contact holes are formed so that the drain of the drain select transistor is exposed. Tungsten (W) is deposited on the interlayer dielectric layer including the contact holes. A Chemical Mechanical Polishing (CMP) process is then performed to form drain contact plugs that gap-fill the contact holes.
In the above process, the thickness of the dielectric layer, which is to be etched when forming the drain contact holes, is too large, resulting in a bowing phenomenon in which the width at the middle of the contact hole is widened. If the bowing phenomenon becomes severe, the contact holes are interconnected, thereby interconnecting the contact plugs formed within the contact holes. To solve this problem, after a metal layer (for example, a tungsten (W) layer) for forming the drain contact plugs is formed on the interlayer dielectric layer including the contact holes, the metal layer and the interlayer dielectric layer are etched to a depth at which bowing is generated. The etch process can be performed using a CMP process.
However, if the tungsten (W) CMP process is performed to form the contact plugs, tungsten (W) is polished about 50 to 80 times faster than that of an oxide layer. This makes it difficult to remove the oxide layer. Therefore, portions where bowing has occurred on the drain contact holes cannot be removed effectively. Thus, a width of the oxide layer, which isolates the drain contact holes from each other after the CMP process is performed, is narrowed, resulting in bridges between the drain contact plugs. Consequently, the process yield and reliability of a device are degraded.